Thermally switched reflective optical shutter

ABSTRACT

The thermally switched reflective optical shutter is a self-regulating “switchable mirror” device that reflects up to 100% of incident radiant energy above a threshold temperature, and reflects up to 50% of incident radiant energy below a threshold temperature. Control over the flow of radiant energy occurs independently of the thermal conductivity or insulating value of the device, and may or may not preserve the image and color properties of incoming visible light. The device can be used as a construction material to efficiently regulate the internal temperature and illumination of buildings, vehicles, and other structures without the need for an external power supply or operator signals. The device has unique aesthetic optical properties that are not found in traditional windows, skylights, stained glass, light fixtures, glass blocks, bricks, or walls. The device can be tailored to transmit sufficient visible light to see through in both the transparent and reflective states, while still providing significant control over the total energy transmission across the device.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/172,156 filed 11 Jul. 2008 entitled “Thermally switched reflectiveoptical shutter,” which is hereby incorporated by reference herein inits entirety. This application is further entitled to the benefit ofpriority pursuant to 35 U.S.C. §119(e) of U.S. provisional patentapplication No. 60/959,096 filed 11 Jul. 2007 entitled “Thermallyswitched reflective optical shutter” and U.S. provisional application61/015,589 filed 20 Dec. 2007 entitled “Thermally switched reflectiveoptical shutter,” each of which is hereby incorporated by referenceherein in its entirety.

BACKGROUND

1. Technical Field

The subject matter described herein relates to a device for controllingthe flow of light and radiant heat through selective reflection. Thetechnology has particular, but not exclusive, application in passive oractive light-regulating and temperature-regulating films, materials anddevices, especially as construction materials.

2. Description of the Related Art

The problem of controlling the flow of radiant energy, e.g., light andheat, in particular in applications such as regulating solar heat gainin buildings and in other applications has previously been addressedusing many optical methodologies. Photodarkening materials have beenused for decades, for example, in sunglass lenses, to selectivelyattenuate incoming light when stimulated by ultraviolet (UV) radiation.When incorporated into windows, such materials can be used to regulatethe internal temperature of a structure by darkening to attenuate brightsunlight, and by becoming transparent again to allow artificial light ordiffuse daylight to pass through unimpeded. Such systems are passive andself-regulating, requiring no external signal other than ambient UVlight in order to operate. However, because they are controlled by UVlight rather than by temperature, such systems are of limited utility intemperature-regulating applications. For example, they may block wantedsunlight in cold weather as well as unwanted sunlight in hot weather.

Electrodarkening materials have also been used to regulate thetransmission of light. The most widely used electrodarkening material isa liquid crystal sandwiched between two highly efficient absorbingpolarizers, which attenuate slightly more than 50% of the light passingthrough them, primarily by absorption. This material is controlled by anelectric field created by coatings of a transparent, electricallyconductive material such as indium-tin-oxide (ITO). These liquid crystalpanels are typically used in video displays, which are designed to notbe isotropic under operating conditions and have seen only very limiteduse in building materials. This is, in part, because of the significantinfrastructure required to utilize them, including electrical wiring andpower sources, and the requirement of either sophisticated controlsystems, sensors, and algorithms, or extensive user inputs, to set thestate of the materials and thus regulate the light, heat, and radiantenergy through them. Electrodarkening and photodarkening materialsattenuate incoming light primarily through absorption rather thanreflection, meaning they will heat up when exposed to bright light. Theheat absorbed by these materials may also offset the reductions inradiative transmission, and thus place significant limits on theirability to regulate temperature.

Wire-grid polarizers (WGPs) which reflect infrared light rather thanabsorbing it, have been used since the 1960s and are described forexample in U.S. Pat. No. 4,512,638 to Sriram, et al. With the advent ofnanoscale lithography in the 1990s and 2000s, it became possible, thoughexpensive, to produce broadband, wire-grid polarizers that reflect invisible and ultraviolet wavelengths, for use with high-end optics andlaser technology as described, for example, in U.S. Pat. No. 6,122,103to Perkins, et al.

More recently, low-cost reflective polarizer films combining theproperties of a layered-polymer distributed Bragg reflector (DBR) with astretched-polymer polarizer have been introduced. Such reflectivepolarizers are used in video displays to enhance brightness byreflecting the attenuated light back into the device rather thanabsorbing it as described, for example, in U.S. Pat. No. 7,038,745 toWeber, et al. and U.S. Pat. No. 6,099,758 to Verrall, et al. Suchreflective polarizers can exhibit specular reflection for onepolarization of light, as in a mirror, or diffuse reflection for onepolarization of light, as in a coating of white paint, or a combinationof the two. These films were developed specifically for the videodisplay market and have not been used outside of it.

In addition, reflective polarizers can be made from certain types ofliquid crystals. Whereas wire-grid polarizers and stretched polymerpolarizers are linearly polarizing, these liquid crystal polarizers(LCPs) are generally circularly polarizing. Thus, light of one helicity(i.e., right- or left-handed) is transmitted and light of the oppositehelicity is reflected.

Thermal switches allow the passage of heat energy in their ON or closedstate, but prevent it in their OFF or open state. These switches aremechanical relays, which rely on contact between two conducting surfaces(typically made of metal) to enable the passage of heat. When the twosurfaces are withdrawn, heat energy is unable to conduct between themexcept through the air gap. If the device is placed in vacuum, heatconduction is prevented entirely in the open state. Another type ofthermal switch involves pumping a gas or liquid into or out of achamber. When the chamber is full, it conducts heat. When empty, thereis no conduction, although radiative transfer across the chamber maystill occur.

Light can be blocked by optical filters which absorb or reflect certainfrequencies of light while allowing others to pass through, thus actinglike an optical switch. Also, the addition of a mechanical shutter canturn an otherwise transparent material—including a filter—into anoptical switch. When the shutter is open, light passes through easily.When the shutter is closed, no light passes. If the mechanical shutteris replaced with an electrodarkening material such as a liquid crystal,then the switch is “nearly solid state,” with no moving parts exceptphotons, electrons, and the liquid crystal molecules themselves. Otherelectrodarkening materials, described for example in U.S. Pat. No.7,099,062 to Azens, et al., can serve a similar function. These opticalfilter/switch combinations are not passive, but must be operated byexternal signals, e.g., electrical signals.

Switchable mirrors are based on reversible metal hydride and metallithide chemistry, described for example in U.S. Pat. No. 7,042,615 toRichardson. These switchable mirrors rely on the physical migration ofions across a barrier under the influence of an electric field andtherefore have limited switching speeds and cycle lifetimes. Inaddition, electrically operated “light valves” combine liquid crystalswith one or more reflective polarizers as described, for example, inU.S. Pat. No. 6,486,997 to Bruzzone, et al. In these devices, the liquidcrystal typically serves as an electrotropic depolarizer, i.e., as astructure that changes or switches the rotation of the polarity of thelight that passes through it on and off under the influence of anelectric field. Some of these devices may be thought of as switchablemirrors, although they are rarely described that way, since theirprimary application is in video displays and advanced optics.

The information included in this Background section of thespecification, including any references cited herein and any descriptionor discussion thereof, is included for technical reference purposes onlyand is not to be regarded as subject matter by which the scope of theinvention is to be bound.

SUMMARY

The technology disclosed herein is directed to the temperature-basedcontrol over the transmissivity of a window or similar material orstructure with regard to radiant energy (e.g., visible, UV, and infraredlight), including the entire range of the solar spectrum, for thepurpose of regulating the flow of heat into a structure based onexternal weather conditions, internal temperature, or any combination ofthe two. This technology may be employed as a device having atemperature-responsive optical depolarizer, for example, a thermotropicliquid crystal) sandwiched between two polarizing filters to regulatethe passage of light energy. The incident energies passing through thisdevice will depend on the reflection and absorbtion efficiencies of thepolarizers used. For example, for polarizers that are very efficient atreflecting radiant energy over the frequency bandwidths of interest, Forexample, up to half of the incident radiant energy passes through thedevice when it is below a threshold temperature and up to 100% of theincident radiant energy may be reflected away from the device above thethreshold temperature, yielding a thermally switched reflective opticalshutter (hereinafter “TSROS” or “shutter”). Lower efficiency polarizers,or polarizers with frequency-dependent efficiencies, may be used toeffect percentages of reflection above and below the thresholdtemperatures that are desirable for aesthetics, energy management, orother reasons. This effect can also be reversed such that the TSROSdevice is reflective in its cold state, or expanded such that thetransmissivity of the TSROS is higher in the transparent state, orretarded such that the reflectivity of the TSROS device is lower in thereflective state.

In one implementation, two reflective polarizing filters which transmitlight of a polarization parallel to their own, and reflect (not absorb)light of a perpendicular polarization are arranged in succession. Whenthe reflective polarizers are oriented in parallel, up to 50% of theincoming radiant energy may be reflected. In practice, a small amount isalso absorbed, so that typically, the light transmission through twoparallel polarizers is 30-40%. When the reflective polarizers areoriented perpendicular to one another, up to 50% of the light is blockedat one polarizer and up to the remaining 50% transmitted by the firstreflective polarizer is blocked by the second reflective polarizer. Inthis case, transmission of light through both reflective polarizers isvery small (often less than 1%) and the majority of the light (oftenclose to 100%) is reflected back in the direction of incidence.

In another implementation, a switchable depolarizer, which changes thepolarization of the light passing through it, is configured inconjunction with two or more polarizers. In one embodiment, theswitchable polarizer may be a liquid crystal sandwiched between twosheets of transparent, microtextured material such as polymer-coatedglass. The switchable depolarizer may be specifically selected ordesigned to be thermochromic, its polarization state shifts at apredetermined temperature. In the “off” state, the polarization state ofincoming light is largely unaffected by the depolarizer, and in the “on”state, light of a particular polarization, having passed through thefirst polarizer, is rotated by a set amount. This is typically done toalign the light with the second polarizer, either in a parallel orperpendicular state depending on the desired optical effect. Thus, thecombination of two reflective polarizing filters and a liquid crystalforms a switchable mirror that reflects either up to 50% or up to 100%of the incoming light, depending on the state of the liquid crystal.

Other features, details, utilities, and advantages of the presentinvention will be apparent from the following more particular writtendescription of various embodiments of the invention as furtherillustrated in the accompanying drawings and defined in the appendedclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

Please note that closely related elements have the same or similarelement numbers in all figures.

FIG. 1 is a schematic, cross section view of one embodiment of a TRSOSdevice depicting a layer of thermally sensitive depolarizer materialsandwiched between two polarizing filters and attached to a transparentsubstrate. The action of incoming light is depicted for a cold state ofthe shutter.

FIG. 2 is a schematic, cross section view of the embodiment of FIG. 1,except that the action of incoming light is depicted for a hot state ofthe shutter.

FIG. 3 is a schematic representation of another embodiment of a TSROSdevice in which the polarizers define apertures or transparent areas toallow some unpolarized light from the external source to pass throughthe shutter without modification.

FIG. 4 is a schematic representation of an additional embodiment of aTSROS device in which an optional color filter has been included foraesthetic or other reasons.

FIG. 5 is a schematic representation of a further embodiment of a TSROSdevice, in which the thermotropic depolarizer has been replaced with, oradditionally serves as, an electrotropic depolarizer, through theaddition of two transparent electrodes and a control system.

FIG. 6 is a schematic representation of an additional embodiment of aTSROS device, wherein the thermotropic depolarizer has been deleted, andthe reflective polarizers themselves are thermotropic. The action ofincoming light is depicted for a cold state of the shutter.

FIG. 7 is a schematic representation of the embodiment of FIG. 6, exceptthat the action of incoming light is depicted for a hot state of theshutter.

FIG. 8 is a schematic representation of an exemplary thermotropicreflective polarizer in both the hot and cold states.

FIG. 9 is a schematic representation of an additional embodiment of aTSROS device, wherein the first polarizer is a polarity-rotatingpolarizer.

FIG. 10 is a schematic representation of an exemplary polarity-rotatingpolarizer, in a cold state.

FIG. 11 is a schematic representation of an exemplary photovoltaicpolarizer.

DETAILED DESCRIPTION

A thermotropic optical depolarizer may be used in conjunction with tworeflective polarizers to create a thermally switched reflective opticalshutter (TSROS) that allows light and radiant energy to pass through theshutter at low temperatures and reflects it away at high temperatures.The depolarizer is specifically selected or designed to be thermotropic,i.e., its polarization state shifts at a predetermined temperature. TheTSROS device has particular, but not exclusive, application inregulating the temperatures of buildings, vehicles, or other structuresby controlling the amount of solar radiation they absorb.

The structure, composition, manufacture, and function of liquidcrystals, polarizers, and reflective polarizers are well documented, butthe following elaboration is presented for better understanding. Manymaterials exhibit thermotropic properties, including liquid crystals,which transition from an ordered or “ON” state (e.g., crystalline,nematic, or smectic) to a disordered or “OFF” state (liquid, isoptropic,or non-polarizing) state at a temperature known as the “clearing point”.For example, 4-butylcyanobiphenyl (CB) liquid crystals have a clearingpoint of approximately 16.5 degrees centigrade, while 6CB liquidcrystals have a clearing point of approximately 29.0 degrees centigrade,and thus “melt” (i.e., become isotropic) under conditions close to roomtemperature. Mixtures of 4CB and 6CB have a clearing point between thesetwo values, in direct, approximately linear, proportion to thepercentage of each component in the mixture. In the “off” state, thepolarization state of incoming light is largely unaffected by thedepolarizer, and in the “on” state, light of a particular polarization,having passed through the first polarizer, is rotated by a set amount(e.g., 45 or 90 degrees, but also 180 or 270 degrees, or other valuesnot divisible by 45).

In some implementations, the TSROS device in a cold (e.g., crystalline,nematic, or smectic) state reflects up to 50% of the light or otherradiant energy that strikes it, and transmits approximately 40%. In ahot (isotropic) state, the TSROS device reflects up to 100% of theincoming light. Thus, it forms a thermally switched, reflective opticalshutter. It may be appreciated that the opposite transition—a shutterthat is reflective when cold and transmissive when hot—is also possible,depending on the exact arrangement of the polarizer and depolarizerlayers.

This technology has particular but not exclusive utility as a buildingor construction material to regulate the flow of radiant energy(including visible, UV, and IR light) through windows, skylights, andother transparent materials based on temperature, thereby restrictingthe admission of radiant energy (e.g., sunlight) at high temperatures.Thus, this technology can be used to regulate the internal temperaturesof buildings and other structures by controlling the amount of solarradiation they absorb.

For the purposes of this document, the term “thermoreflective” is usedherein to describe a device or material with variable reflectivity thatvaries with or is directly controlled by temperature. The term “radiantenergy” is used to refer to visible light, infrared, ultraviolet, radio,microwave, radar, and other wavelengths of electromagnetic radiationthat obey the laws of optics. Similarly, whenever the terms “light” oroptical” are used herein, they are intended to encompass any form ofradiant energy. The term “optical” as used herein refers to any effectof a material or device on radiant energy, for example, absorption,reflection, transmission, polarization, depolarization, or diffusion.

For the purposes of this document, the term “thermotropic depolarizer”means a material in which the depolarization, e.g., rotation ofpolarization, varies with or is directly controlled by temperature. Oneway to construct a thermotropic depolarizer is to hold thermotropicliquid crystal between two alignment layers. The orientations of thethermotropic liquid crystal molecules are influenced both by thealignment layers, e.g., their chemistry and structure, and thetemperature or temperature gradient. In a thermotropic liquid crystalwhich has a nematic state, this structure can be utilized as a waveblockwhere the rotation of polarization of various frequencies and bandwidthsof light are temperature dependent, and where the crystal-like structureof the waveblock collapses above a threshold temperature. Note that thisdiscussion of thermotropic liquid crystals is provided as an example andshould not be considered as limiting the scope of the TSROS device.

For the purposes of this document, the term “switch” includes bothsolid-state and mechanical devices for selectively blocking orpermitting the flow of energy, and includes both digital switches (e.g.,transistors and relays) and analog regulators (e.g., tubes andrheostats). Furthermore, a valve for selectively blocking or regulatingthe flow of gases or fluids can be considered analogous to a switch sothat, in principle, the two terms can be used interchangeably. By thisdefinition, the TSROS device is a solid-state optical switch, whichmoves from its “open” or transmissive state to its “closed” orreflective state based on the temperature of the TSROS device.

For the purposes of this document, the term “passive” refers to anobject or device that responds to environmental conditions but operatesindependently of external signals or instructions from an operator.Thus, a device may include a number of complex components—even movingparts—and still be regarded as “passive” for the purposes of thisdocument. Similarly, the possible existence of a user override mode doesnot alter, in any essential way, the passive nature of such a device. Bycontrast, an active device is one that requires user input in order toperform its normal functions. As an example, these definitions,light-sensitive sunglasses are a passive device, whereas a standardlight bulb operated by a wall switch or dimmer switch is an activedevice.

For the purposes of this document, the term “depolarizer” refers to anobject, device, or substance that rotates or otherwise alters thepolarization vector of light passing through it in some way other thanattenuation. Separately, the term “polarizer” refers to an object,device, or substance that blocks light of one polarity whiletransmitting light of orthogonal polarity or, in the case of circularlypolarized light, of opposite helicity. Most typically, this blockingoccurs by absorption. For the purposes of this document, the term“reflective polarizer” refers specifically to a polarizer that blockslight of one polarity by reflecting it rather than by absorbing it. Bythis definition, a standard absorptive polarizing filter adjacent to astandard reflecting or semi-reflecting filter is not a reflectivepolarizer and should not be confused with one.

It should also be understood that some absorption occurs in reflectivepolarizers, just as some reflection occurs in absorptive polarizers, butthat the distinction between reflective-type and absorptive-typepolarizers is significant, as the two types rely on different operatingprinciples and produce qualitatively different optical effects. Whendiscussing reflective polarizers, it is convenient to assume forpurposes of exemplary discussion that they are 100% efficient (orapproximately 100% efficiency) in reflecting light of one polarity andtransmit the other polarity of light. However, in actual practice, thesepolarizers may be less than 100% efficient (e.g., due to design choiceor design and manufacturing limits), be partially absorptive, and havefrequency-dependent and spacially dependent reflection, absorption, andtransmission characteristics (e.g., due to design choice or design andmanufacturing limits) and this should not be construed as limiting thescope of the invention.

FIG. 1 is a schematic, cross section view of one embodiment of a TSROSdevice depicting a depolarizer layer 102 sandwiched between tworeflective polarizing filters 101 and 103, and attached to an optionaltransparent substrate 104. In the most general case the external lightsource will be unpolarized white light (i.e., light with significantintensity across a significant bandwidth of the visible, near-UV andnear-IR spectrum). In one exemplary use of the device, the externallight source is the sun. However, the device will also function when theexternal light source is not white, as for example a street lamp or thediffuse radiant energy of the blue sky.

Incoming light first passes through the outer reflective polarizer 101.Exemplary forms of the reflective polarizer 101 include a wire gridpolarizer composed of a microscopic array of metal wires affixed to orembedded within a transparent substrate such as glass or plastic, or apolymer-based reflective polarizing film, or a liquid crystal polarizer(LCP), although other forms may also be used. Note that wire gridpolarizers have the property of polarizing across an extremely broadrange of wavelengths, including radio, microwave, and radar wavelengths,which may be particularly useful in some applications.

Of the incoming light, approximately 50% will have polarizationperpendicular to that of the polarizer 101, and will likely be reflectedaway. By contrast, an ordinary absorptive polarizer will absorb lightwith perpendicular polarization, rather than reflecting it, and willconsequently heat up. Of the remaining light with polarization parallelto that of the reflective polarizer 101, some percentage is absorbed,and the remainder is transmitted through.

Once it has passed through the outer reflective polarizing filter 101,the incoming light (e.g., sunlight) enters the thermotropic depolarizer102, which is a device or material capable of exhibiting two differentpolarizing states. In its hot or isotropic or liquid state, thepolarized light passing through it is not affected. In its cold (e.g.,nematic or crystalline) state, the thermotropic depolarizer 102 rotatesthe polarization vector of the incoming light by a fixed amount. In thepreferred embodiment, the depolarizer 102 is a twisted nematic liquidcrystal that rotates the light's polarization vector by 90 degrees.However, a variety of other devices and materials are capable of servingas well, including nematic liquid crystals oriented at 45 degrees, or atsome other angle, to the outer reflective polarizer 101.

Once it has passed through the thermotropic depolarizer 102, theremaining polarized light strikes the inner reflective polarizer 103,also known as the “analyzer”, where it is either reflected ortransmitted, depending on its polarization state. The inner reflectivepolarizer 103 is oriented such that its polarization is perpendicular tothat of the outer reflective polarizer. Thus, in the device's hot state,when the light's polarization vector has not been rotated, the light'spolarity is perpendicular to that of the inner reflective polarizer 103,and up to 100% of it is reflected. However, in the cold state, when thelight's polarization vector has been rotated by 90 degrees and isparallel to the inner reflective polarizer 103, some of the light isabsorbed by the polarizer material, and the rest is transmitted through.

The action of incoming light is depicted for the device's cold state:the outer reflective polarizer 101 reflects up to 50% of the incominglight. The remaining light passes through the thermotropic depolarizer102, where its polarization vector is rotated, and then through theinner reflective polarizer or analyzer 103, where it is largelyunaffected. It then passes through an optional transparent substrate104, and finally exits the device. Thus, in its cold state the deviceserves as a “half mirror” that reflects up to 50% of the light strikingits outer surface, absorbs a small amount, and transmits the restthrough to the inner surface.

FIG. 2 is a schematic, cross section view of the embodiment of FIG. 1,except that the action of incoming light is depicted for a hot state ofthe shutter. The thermotropic depolarizer 102 does not affect thepolarization vector of the light passing through it. Thus, any lightstriking the inner reflective polarizer is of perpendicular polarity toit, and up to 100% is reflected back. The TSROS device therefore servesas a “full mirror” that reflects up to 100% of the light striking itsouter surface.

Thus, in its cold state the shutter transmits slightly less than halfthe light energy which strikes its outer surface, whereas in the hotstate the shutter transmits substantially less than 1% of the lightenergy. As a result, the shutter can be used to regulate the flow oflight or radiant heat into a structure based on the temperature of theshutter.

From the above description, a person of ordinary skill in the art willrealize that in this embodiment, the transparent substrate 104 ispresent only for reasons of structural support and convenience. Thiscomponent may be deleted without significantly altering the function ofthe shutter. Alternatively, the transparent substrate 104 could beplaced on the outer surface of the shutter rather than the innersurface, or transparent substrates 104 could be placed on both surfaces,or even inserted between one or more of the functional layers of theshutter, without significantly altering its function. Furthermore, ifthe transparent substrate 104 is located on the inside surface of theshutter as shown in FIGS. 1 and 2, it need not be transparent to allwavelengths, and can in fact be a longpass, shortpass, or bandpassfilter as long as the transmitted wavelengths are useful as heat energy,illumination, or for some other purpose. However, for convenience andcost it will generally be preferable to use an ordinary transparentmaterial such as glass or acrylic as the substrate.

Because the eye works on a logarithmic scale, preliminary evidenceindicates that a 50% attenuation of incoming light will appear,subjectively, to be approximately 84% as bright as the original,unattenuated light, but may vary. As a balance of aesthetic, human, andenergy management factors, preliminary evidence indicates a hot statetransmission of approximately 10-20% of incident solar energy, and acold state transmission of 50-70% of incident solar energy are desirablefor window applications. Different transmissivity levels may thus bedesirable for different uses and embodiments of the TSROS device

In one exemplary process for fabricating a TSROS device, the first stepis to create the liquid crystal (LC) cell or “bottle”. Two sheets ofSiO₂-coated (passivated) glass are scribed to a pre-determined size andplaced in substrate holders. If there is an indium tin oxide (ITO) lowemissivity coating on the glass, it should be etched off, leaving theSiO₂ in place. The sheets are then placed in a 48 KHz ultrasonic cleaner(e.g., Crest Truesweep set at power level 8) for 15 minutes, using a pHneutral soap mixed at 1 oz per gallon of deionized (DI) water (28 Ohmpurity or better). If there are polyimide (PI) wetting issues then thesheets may be rewashed with Detrex soap. Larger sheets may be cleanedinstead using a commercial glass washer (e.g., Billco Series 600). Thesheets may be dried with isopropyl alcohol (IPA) and placed in a dryingoven at 80-85 C for 120 minutes or longer as needed for moisture-freestorage and staging, and are then placed in an ozone cleaner for 15minutes. A PI alignment layer, dissolved in a solvent, is then depositedby spin coating at 500 RPM for 10 seconds followed by 2000 RPM for 45seconds. Consistent coating requires approximately 1 ml per square inchof Sheet. For sheets too large to spin coat, the PI solution isdeposited by inkjet printer. After coating, the substrates are heated to85 C for 5 minutes to flash away any remaining solvent, and then bakedat 180-190 C for 1 hour to harden the PI. The oven door should not beopened until the inside temperature is 85 C or lower.

To prevent contamination of the PI surface, sheets are then stored in avacuum oven at 50 C until needed. The sheet is then placed in a vacuumfixture to hold it in place, and rubbed with a block of polypropylene oraluminum wrapped with rub cloth material secured with double-sided tape.The rub block is pushed across the surface 25 times in the samedirection with no downward pressure other than its own weight. The rubdirection is then marked (e.g., with a Sharpie pen) on the uncoated sideof the sheet. A plurality of 7.5-micron spacer beads are then applied tothe rubbed surface of one sheet with an air puff machine, and a secondsheet, with rub direction oriented at 90 degrees from the firstsubstrate, is placed rubbed-side-down atop the first substrate. Theedges are sealed first with an optical adhesive (e.g., Norlin 68), whichdoes not interact with the liquid crystal, and then with a waterproofsealer (e.g., Loctite 349), leaving at least two ports open, eachapproximately 1 cm wide. The Norlin 68 is then UV cured with a dose ofat least 4000 mJ/cm² and either baked for 12 hours at 50 C or elseallowed to cure at room temperature for a full week.

The bottle is then placed in a vacuum loader with a pressure of 20milliTorr or less and at a temperature below the clearing point andabove the freezing of the liquid crystal, and lowered into a slotcontaining the liquid crystal (e.g., a mixture of 5 parts 6CB, 1.25parts E7 and 0.008 parts 811 with a clearing point of 35 C). The liquidcrystal is drawn into the bottle by capillary action. When loading iscomplete, the bottle is removed from the vacuum chamber, the ports aresealed with Norlin 68 and Loctite 349, and the curing step is repeated,taking care to avoid unnecessary exposure of the liquid crystal mixtureto UV light. The bottle is now complete.

Once a bottle is fabricated, it can then be further constructed into afinished TSROS device. Exemplary TSROS devices include a stand-alone,thermoreflective filter (e.g., an LC bottle, polarizers, and UVprotection only) and an insulated glass unit (IGU) or “double-panedwindow” with the thermoreflective filter laminated to one pane. Tofabricate a thermoreflective filter, the LC bottle is laminated severaltimes with layers of optically clear sheet adhesive (e.g., 3M 8141 and3M 8142 optically clear adhesive), and reflective polarizer films (e.g.,3M advanced polarizing film (APF) or diffusive polarizing reflectivefilm (DRPF)). A layer of UV shielding is then applied (e.g., GamColor1510 UV film). All lamination steps are performed in a class 10,000cleanroom environment with a class 1,000 downdraft hood to preventparticulates from causing air bubbles in any of the adhesive layers.

The process begins by using a 6 ft automatic/manual roll laminator tobegin applying adhesive to the bottle. Using preset increments on theleveling knobs, an elevation is set on the laminator to avoid damagingthe bottle. One layer of 3M 8141 is applied to the bottle, followed by alayer of either APF or DRPF. The process is then repeated on the reverseside of the bottle, with the polarizer film at 90° rotation from theprevious layer. One more layer of 3M 8141 is applied to either side ofthe bottle, and then a layer of UV shielding is applied as the laststep. At this point, the bottle has become a thermoreflective filter.

In order to fabricate an insulating glass unit (IGU) from the LC bottle,further lamination is required. The thermoreflective filter is given twoconsecutive layers of 3M 8142 over the UV shielding. Tempered glass,typically larger than the bottle by 1-2 in, is then also given twoconsecutive layers of 3M 8142. The layers on the tempered glass arelaminated with tape underneath the border to prevent the 3M 8142 fromcompletely covering the glass. The adhesive backing is then removed fromboth the LC bottle and the tempered glass pane. The adhesive sides ofeach are placed together and then run through the laminator a finaltime, again set at an elevation that is suitable for lamination andprevents damaging the bottle. The assembly can now have a standardinsulated glass unit built around it. Standard aluminum spacers withdesiccant are used to separate the two panes of IGU glass and areattached to the glass with PIB bonding beads and sealed around the edgeswith polyisobutylene (PIB) hot-melt sealant. The IGU is now ready forshipping and installation.

FIG. 3 is a schematic representation of another embodiment of a TSROSdevice, in which there are gaps 105 in one or both polarizers 101 and103 to allow some unpolarized light from the external source to passthrough the shutter without modification. These gaps 105 may take theform of holes or stripes, or alternatively the polarizer material itselfmay be applied in stripes or spots. However, a person of ordinary skillin the art will understand that there are numerous alternate methods forfashioning the gaps 105 that need not be elaborated here. Thisembodiment may be useful, for example, in windows that are required tooffer a relatively clear, unattenuated view. In this case, theattenuation or obstruction of the polarizers 101 and 103 in thereflective state would be similar to looking through a normal windowscreen.

The use of polarizers 101 and 103 with gaps 105 in place of uniformpolarizers increases the transmission of energy through the shutterunder all conditions, and thus reduces the ability of the shutter toreflect light and radiant energy in its hot state. However, thisarrangement may be advantageous under circumstances where cold-statetransparency is more important than hot-state reflectivity.

It may be noted that a similar effect can be achieved by rotating thetwo polarizers with respect to one another, to an angle greater thanzero and less than 90 degrees, although this method can only increasetransparency in the hot state of the shutter, and may (depending on theexact geometry of the shutter and the exact properties of thedepolarizer) actually decrease transparency in the cold state. Assumingthe polarizer is operating efficiently, the transparency in the coldstate can never be greater than 50%—the state that occurs when two idealpolarizers are placed in parallel orientation. However, greatertransparency can be achieved in the cold state—at the expense ofreflectivity in the hot state—if the polarization efficiency is lessthan 100%.

Also note that gaps in, or other alterations to, the liquid crystalalignment layer can produce an effect similar to having gaps in thepolarizer, and that under some circumstances this may be easier orotherwise more desirable. Also, gaps 05 of any sort can be arranged sothat only indirect light is able to pass through the shutter.

FIG. 4 is a schematic representation of an additional embodiment of aTSROS device in which an optional color filter 106 has been added.Exemplary forms of the color filter 106 may include a band reflector(such as a distributed Bragg reflector (DBR) or rugate filter), which isdesigned to reflect a narrow range of wavelengths and transmit allothers, or a bandpass filter (e.g., a sheet of colored glass orplastic), which is designed to transmit a range of wavelengths andreflect or absorb all others.

The color filter 106 is depicted as being on the exterior surface of theshutter. However, a person of ordinary skill in the art will understandthat different aesthetic or optical effects could be created by placingthe color filter 106 behind other layers in the shutter. For example, ifthe color filter 106 were placed on the inner surface of the shutter,then the color would not be apparent to an exterior observer when theshutter was in its hot, or 100% reflective, state.

The use of a color filter will reduce the amount of light and radiantenergy transmitted through the shutter in its cold, or 50% reflective,state. However, this arrangement may be advantageous under circumstanceswhere aesthetics, rejection of key wavelengths, or hot-statereflectivity are considered more important than cold-state transparency.

Alternatively, instead of an additional color filter layer, the shuttercan be used with one or more colored polarizers (i.e., one which doesnot absorb or reflect across the entire visible spectrum) in place ofone of the reflective polarizers. One exemplary colored polarizer is the3M DBEF reflective polarizing film, which yields a magenta color (acombination of red and blue) in the hot or reflective state.

FIG. 5 is a schematic representation of a further embodiment of a TSROS,in which the thermotropic depolarizer 102 has been replaced with, oradditionally serves as, an electrotropic depolarizer 102′, plus twotransparent electrodes 107 and a control system 108, which collectivelyperform the same function.

An exemplary form of the transparent electrodes is a thin layer ofindium tin oxide (ITO). The control system 108 includes a temperaturesensor, power supply, and controller hardware. An exemplary form of thecontrol system 108 is a thermostat and LCD controller consisting of athermocouple connected to a programmable microcontroller and powered bya small battery or photovoltaic cell. When the sensed temperature fallsbelow a threshold value, the control system applies an AC or DC voltagebetween the transparent electrodes 107 that creates an AC or DC electricfield across the electrotropic depolarizer 102′, such that itspolarization properties are altered (e.g., by reorienting liquid crystalmolecules). The design of such control systems is commonplace in theprior art and needs no detailed elaboration herein. The operation anduse of this embodiment are otherwise identical to operation and use ofthe embodiment shown in FIGS. 1 and 2.

FIG. 6 is a schematic representation of an additional embodiment of aTSROS device, wherein the thermotropic depolarizer 102 has been deleted,and the reflective polarizers 101′ and 103′ are thermotropic. The designof the thermotropic reflective polarizers 101′ and 103′ is such thatthey polarize normally in the hot state, and are minimally polarizing ornonpolarizing in the cold state. Thus, in the cold state, unpolarizedlight entering the shutter encounters the outer polarizer 101′ in itsnonpolarizing state, and is not significantly affected by it, and thenencounters the inner thermotropic reflective polarizer 103′ in itsnonpolarizing state, and is not significantly affected by it either.Thus, except for some minor absorption, reflection, and scatteringassociated with transmission through the transparent substrate and thethermotropic reflective polarizers in their nonpolarizing state,essentially 100% of the incoming light is transmitted through theshutter.

FIG. 7 is a schematic representation of the embodiment of FIG. 6 in ahot state. In this case both thermotropic reflective polarizers 101′ and103′ are in their fully polarizing configuration, with no depolarizerbetween them. Thus, when unpolarized light encounters the outerthermotropic reflective polarizer 101′, up to 50% of it is reflected asin the other embodiments. The light that passes through is of oppositepolarity, and therefore up to 100% of it is reflected. Thus, in its coldstate the shutter is up to 100% transmissive, and in its hot state it isup to 100% reflective. As in other embodiments, this ideal reflectionoccurs when the two thermotropic reflective polarizers 101′ and 103′ areoriented 90 degrees apart; the amount of transmission and reflection inthe hot state may be adjusted by misaligning the two thermotropicreflective polarizers 101′ and 103′, and the amount of transmission andreflection in the cold state can be adjusted by placing a depolarizerbetween the thermotropic reflective polarizers 101′ and 103′.

FIG. 8 is a schematic representation of an exemplary thermotropicreflective polarizer 101, in both its hot and cold states. In thisexemplary embodiment, the polarizer 101 is a wire grid polarizercomposed of parallel metal wires 109. However, unlike standard wire gridpolarizers, the polarizer in this embodiment is a MEMS(microelectrical-mechanical systems) device with wires 109 composed ofwire segments 110 made of a conductive, thermotropic material thatchanges its physical shape in response to temperature. Examples of suchmaterials include, but are not limited to, shape memory alloys such ascopper-aluminum-nickel alloy. In this exemplary embodiment, the wiresegments 110 are formed such that they lie flat at high temperatures,and stand up away from the surface at low temperatures. Thus, above acertain threshold temperature, the individual wire segments 110 lie flatenough to come into physical contact with one another and formcontinuous wires 109, which collectively form a wire grid polarizer 101or 103. However, it may be understood that other forms of thermotropicreflective polarizers are also possible, including versions composed ofliquid crystals or nanoengineered optical and photonic materials orso-called “metamaterials”, and that these or other forms of thermotropicreflective polarizer can be employed in place of the design shown inFIG. 8 without altering the fundamental nature, intent, or functioningof this implementation.

FIG. 9 is a schematic representation of still another embodiment of aTSROS device, in which the outer reflective polarizer 101 has beenreplaced with a “polarity-rotating polarizer.” Whereas an absorptivepolarizer absorbs light of opposite polarity and a reflective polarizerreflects light of opposite polarity, a polarity-rotating polarizerconverts light of opposite polarity into light of matching polarity.Thus, the polarizer 101 is up to 100% transmissive, and all the lightthat exits it has the same polarity. In the figure, incoming light ofmatching polarity strikes the outer polarizer 101 and is transmittedthrough. Light of opposite polarity strikes the outer polarizer 101 andis “rotated” so that its polarity matches that of the polarizer.

FIG. 9 depicts the operation of this embodiment in the cold state: thepolarized light then enters the depolarizer 102, which is in its cold,organized state (e.g., a twisted nematic state) and thus functions torotate the polarity of all the light passing through it, to match thepolarity of the second polarizer or analyzer, 103, which is a standardreflective polarizer as in other embodiments previously described. Sincethe depolarized light matches the polarity of the second polarizer 103,it is transmitted through. Therefore, in this embodiment the TSROSdevice is up to 100% transmissive in the cold state. In the hot state,the depolarizer 102 becomes disorganized (i.e., the liquid or isotropicstate) and does not affect the polarity of the light passing through it.Therefore, since the light is of opposite polarity to the secondpolarizer 103, up to 100% of the light is reflected back. Thus, theTSROS device is up to 100% reflective in its hot state.

FIG. 10 is a schematic representation of an exemplary form of apolarity-rotating polarizer device, consisting of a wire grid polarizer111, a mirror 112, and a depolarizer 113. When light of matchingpolarity strikes the polarizer 111, it is transmitted through. However,when light of opposite polarity strikes the polarizer 111, it isreflected at a 45-degree angle to the mirror 112, which also reflects itat a 45-degree angle such that the light is traveling once again in itsoriginal direction. At this point, the reflected light passes through apermanent depolarizer (also known as a waveblock or waveplate) thatrotates its polarity by a specific amount (usually 90 degrees). Thus,the polarity of the reflected light now matches the polarity of thetransmitted light. Therefore, the polarity-rotating polarizer transmitsup to 100% of the light that strikes it, while ensuring that all of thelight is of the same polarization.

It should be appreciated that myriad other arrangements of opticalcomponents can achieve the same effect, and that other types ofpolarity-rotating polarizers may be discovered as well, includingpolarizer materials based on nanostructured optical or photonicmaterials, so-called “metamaterials”, and other materials that functionon different principles. However, the basic functioning, intent, andperformance of the present implementation is not affected by the exactnature of polarity-rotating polarizer employed.

FIG. 11 is a schematic representation of still another type ofreflective polarizer, a photovoltaic polarizer, wherein, the conductivewires 109 of a wire grid polarizer 111 have been replaced withphotovoltaic strips. In the preferred embodiment, these strips 109 areShotkey-type diodes consisting of a thin film of metal (e.g., aluminum)on top of a thin film of semiconductor (e.g., silicon). However, otherphotovoltaic materials or devices could be substituted with no essentialchange to the nature or functioning of this embodiment of a TSROSdevice. In this arrangement, while the photovoltaic strips 109 reflect asignificant fraction of the light that hits them with opposite polarity,as with an ordinary wire grid polarizer, a significant fraction of thislight is also absorbed in the form of electrical potentials which can beharvested to create an electrical current. The design and functioning ofphotovoltaic devices is well described in the prior art and needs nofurther elaboration here.

However, it should be understood that one or more photovoltaicpolarizers can be employed in the present implementation, such that aportion of the light blocked by the polarizer or polarizers can beexploited in the form of electrical power. This occurs in addition tothe normal thermoreflective behaviors of the shutter.

A TSROS device is passive, self-regulating—requiring no external signalsor user inputs in order to function and thus may be considered aso-called “smart material.” The TSROS device may also be understood as anearly-solid-state optical switch. In some implementations, aside from athin film of liquid crystal molecules, the switch contains no movingparts, other than photons and electrons. The TSROS device regulates,based on temperature, the amount of light and radiant energy that passesthrough it. The shutter can thereby be used to help regulate theinternal temperatures of buildings, vehicles, and other structures bycontrolling the reflection and absorption of solar energy or otherincident light energy

The TSROS device may have multiple configurations. For example, if theTSROS device is configured to transmit and reflect diffuse visiblelight, it may serve as an aesthetic, energy-regulating replacement fortranslucent building materials such as glass block, privacy glass, andtextured polymers. Alternatively, if the TSROS device is configured totransmit and reflect collimated visible light with little diffusion, itserves as an aesthetic, energy-regulating replacement for transparentbuilding materials such as glass or polymer windows. Further, if theTSROS device is configured to show reflection or transmission peaks inthe visible spectrum, it may serve as an energy-regulating replacementfor stained glass, tinted windows or window appliqués and coatings, orcolored artificial light sources. The physical instantiation of a TSROSdevice may be thick or thin, strong or weak, rigid or flexible,monolithic or made up of separate parts, without altering its basicfunction in any significant way.

While several exemplary embodiments are depicted and described herein,it should be understood that the present invention is not limited tothese particular configurations. Optional components such asantireflective coatings or films may be added or moved to suit the needsof a particular application or a particular manufacturing method, anddegraded forms of some embodiments can be produced by deleting orsubstituting certain components. For example, replacing one, (but notboth, of the reflective polarizers with an ordinary absorptive polarizerwould result in a TSROS device that is up to 50% reflective, 40%transmissive, and 10% absorptive in its cold state, and up to 50%reflective, 50% absorptive, and less than 1% transmissive in its hotstate. Such a TSROS device would absorb heat in its hot state, and thuswould not block heat as effectively. Nevertheless, this arrangementmight be advantageous if, for example, the cost of the TSROS device isconsidered more important than its performance.

In addition, it is possible to enhance the performance of a TSROS deviceby improving its cold-state light transmission, reflection, orabsorption, by increasing or decreasing its thermal conductivity and/orthermal capacity. Furthermore, it is possible to increase thetransparency of the TSROS device in both the cold and hot states byadjusting the structure of one or both polarizing layers (e.g., byalternating stripes or spots of polarizer material with transparentmaterial). It is possible to increase or decrease the transparency ofthe TSROS device in the hot state, though not in the cold state, byadjusting the orientation of the polarizers with respect to one another(i.e., less than 90 degrees will produce less than 100% reflectivity inthe hot state). It is also possible to increase or decrease thetransparency of the TSROS device in the cold state by adjusting therotation of polarized light provided by the depolarizer. The TSROSdevice is also functionally enhanced for certain applications throughthe addition of optional features such as anti-reflection coatings,low-emissivity coatings, concentrating lenses, air gaps or vacuum gaps,phase change materials, or translucent thermal insulators including butnot limited to foamed glass and silica aerogels.

Various polarizer types (including but not limited to wire gridpolarizers, stretched polymer polarizers, liquid crystal polarizers,absorptive polarizers, specular reflective polarizers, diffusereflective polarizers, thermotropic polarizers whose function changeswith temperature, and polarity-rotating polarizers) can be combined inan enormous number of different arrangements to achieve effects similarto those described in this application, without fundamentally deviatingfrom the spirit of the invention. For example, the reflective polarizersmay be selected such that each has a different polarizing nature, e.g.,the polarizing natures could be opposing as specular vs. diffusive orreflective vs. absorptive, at different frequencies.

Although the maximum control over energy transport for the TSROS deviceoccurs when the range of reflected wavelengths is as large as possible,the addition of color filter layers can alter the transmission spectrum(i.e., the color) of the light passing through the TSROS device, foraesthetic or other reasons. In addition, it is possible to add areflective “color” to the surface of the TSROS device, with minimaleffect on its efficiency, by adding one or more bandblock filters toreflect particular wavelengths of light. The resulting opticalproperties do not closely resemble those of any other building material,although they may bear a passing resemblance to certain types ofsunglasses. It is also possible, for aesthetic, heat and lightmanagement, or other reasons, to use spectrally selective polarizersthat operate only over a particular range (or multiple ranges) ofwavelengths, as well as polarizers which have different polarizationefficiencies and degrees of absorptivity and reflectivity overparticular ranges (or multiple ranges) or wavelengths.

Although the materials and structures of a TSROS device may be rigid,there is no requirement for rigidity in order for it to perform thefunctions described herein. Furthermore, while the various components ofthe TSROS device are shown and described as being attached or in directphysical contact, the TSROS device will also function if the componentsare merely adjacent but physically separate. Thus, while the TSROSdevice can be embodied as a solid object (e.g., a window, glass block,spandrel, or movable panel) or group of solid objects (e.g., componentsaffixed to an optical workbench), it can also be embodied as a flexibleobject such as, for example, a tent material, blanket, curtain, or anappliqué film which can be applied to the surface of glass windows,spandrels, or glass block building materials.

In addition, a wide variety of alternative materials can be used tomanufacture the device, including, metals, ceramics, semiconductors,glasses, polymers, nanostructured and microstructured photonicmaterials, metamaterials, liquid crystals, and even ices, liquids, andvapors. The device may include features designed to enhance its thermalinsulation properties including but not limited to air gaps, vacuumgaps, foams, beads, fiber pads, or aerogels. It may also includefeatures designed to improve thermal sensing, response, and transitiontemperature accuracy capabilities, such as conductive adhesives,materials with large or small thermal masses, and phase changematerials. It may be thick and rigid enough to serve as a structuralcomponent of vehicles or building walls. It may be wrapped around orformed upon complex surfaces. It may be aesthetically enhanced withcolor, or it may be camouflaged to resemble more conventional buildingmaterials. Thermochromic pigments may be added to certain surfaces toindicate when they are hot or cold.

Additives, e.g., chiral liquid crystal may be included in thethermotropic depolarizer to set a preferred direction of rotation ofpolarized light. This may improve the speed and optical properties ofthe transition between states. A solvent (e.g., Merck liquid crystalsolvent ZLI1132) may be used as a base to create a mixture of liquidcrystals. Additionally, additives may be included in the depolarizer,for example, to improve the temperature stability of transitions or toreduce the suceptability of the depolarizer to light or energy ofparticular wavelengths or ranges of wavelengths to reduce chemicalsusceptibility to breakdown due to UV light, to prevent the absorptionof certain wavelengths of light and conversion into heat, or to mitigatechanges in transition temperatures due to chemical breakdown of othercomponents). For example, hexane and chloroform may be introduced toadjust the freezing temperature or lower viscosity. Mechanicalenhancements may be added to reorient components, either to face themtoward or away from incoming light, or to alter their wavelengthresponse or apparent thickness.

The exact arrangement of the various layers can be different than isdepicted here, and (depending on the materials and wavelengths selected)different layers can be combined as single layers, objects, devices, ormaterials, without altering the essential structure and function of aTSROS device. Although the description above contains manyspecificities, these should not be construed as limiting the scope ofthe invention but rather construed as merely providing illustrations ofcertain exemplary embodiments of this invention. There are variouspossibilities for making the TSROC device of different materials, and indifferent configurations. For example, the structure could be inflatableor could be optimized for use underwater or in outer space instead of innormal air.

In addition, the TSROS device could incorporate one or more additionalpolarizers, whether parallel or rotated at some angle to one another andto the original two polarizers, in order to modify the polarizationstate of light at various positions within the TSROS device. In anembodiment incorporating multiple polarizers, not all of the polarizerswill necessarily be reflective polarizers, although at least one mustbe. Numerous combinations of polarizer angle and liquid crystal moleculeorientation can be used to produce different optical effects (e.g.,reflective when cold instead of reflective when hot, different colors inthe transmissive state, etc.). The depolarizer can employ a wide varietyof different combinations of nematic, twisted nematic, smectic,solid/crystalline, discotic, chiral, and other physical/molecularstates, as well as alternative liquid crystal technologies such aspolymer stabilized cholesterics, and guest-host cells, with or withoutelectric fields, textured surfaces, internal guide wires, or other meansto reorient the component molecules.

It is possible to make depolarizers which are diffusive in the coldstate and specular in the hot state (and reverse), are opalescent in oneor both states, change the color balance of the transmitted andreflected light differently as the temperature changes, and similarlyhave different color balances when in the hot and cold state. Throughthe use of lenses, prismatic films, directionally sensitive polarizers,or non-parallel orientation of reflective components, reflections fromthe TSROS device can be sent in any direction, or can be diffused tolimit the blinding “second sun” effect that sometimes occurs near large,mirrored surfaces. Additionally, it is possible to use polarizers thatact on any of the various polarizations of light, (e.g. circular,elliptical, and linear). Such embodiments are explicitly claimed as partof the present invention.

Numerous other variations exist which do not affect the core principlesof the invention. For example, the depolarizer or one or more polarizerscould be mechanical in nature, physically rotating by 90 degrees (or bysome other amount) in response to a shift in temperature. Alternatively,the thermotropic depolarizer could be designed such that its temperatureaffected the range of wavelengths over which it operated, rather than(or in addition to) affecting its ability to depolarize. For example, awaveblock made from a material with very high coefficient of thermalexpansion would have this property. Any or all of the layers in thedevice could be composed of doped, nanostructured, or microstructuredmaterials including but not limited to custom photonic crystals. One ormore layers could be non-planar in shape (e.g., parabolic mirrors formedfrom reflective polarizers), or other shaped reflectors or similardevices could be incorporated, to help concentrate, diffuse, orotherwise affect incoming light from a variety of angles.

The use of a TSROS device as a thermally-regulating building materialmay be enhanced by careful positioning of the device, for example byplacing it under the eave on the south face of a house so that thedevice is in full sunlight during winter days and is shadowed by theeave on summer days when the sun is higher in the sky. Alternatively, itcan be used in place of traditional skylights, or as a panel or appliquéaffixed to ordinary glass windows or glass blocks. In some cases, it mayalso be desirable to place opaque, energy-absorbing materials behind athermoreflective material or device in order to maximize the absorptionof heat energy in the cold (transparent) state.

While a TSROS device as disclosed herein has particular application as abuilding material, particularly for the exterior skin of structuresexposed to sunlight, it can be used in myriad other ways as well. Forexample, a thermoreflective material or device could be incorporatedinto shower doors such that the presence of hot water or steam causesthe door to become reflective, guaranteeing the privacy of the occupant.Similarly, a coffee pot could be made thermoreflective, such that thepresence of hot coffee in the pot would be obvious to any observer.

In addition, a TSROS device can be used to displaytemperature-controlled reflective images. Such images, including text,line drawings, corporate logos, and monochromatic photographs, can beproduced by arranging thermoreflective materials in the shape of thedesired image, or by selectively varying the temperature response of thethermoreflective materials in particular areas so that the image appearsat particular temperature or range of temperatures, or by manipulatingliquid crystal alignment layers or other molecular alignment processessuch that the material's thermoreflective response is enhanced orreduced in particular areas to form the image, or by other methods whichdo not fundamentally alter the nature of the image or its underlyingtechnology. Such images can include reflective optical components suchas mirrors, half-mirrors, gratings, grids, and fresnel lenses, such thatthe thermoreflective material or device exhibits markedly differentoptical properties at high temperature than at low temperature.

Although various embodiments of this invention have been described abovewith a certain degree of particularity, or with reference to one or moreindividual embodiments, those skilled in the art could make numerousalterations to the disclosed embodiments without departing from thespirit or scope of this invention. It is intended that all mattercontained in the above description and shown in the accompanyingdrawings shall be interpreted as illustrative only of particularembodiments and not limiting. All directional references e.g., proximal,distal, upper, lower, inner, outer, upward, downward, left, right,lateral, front, back, top, bottom, above, below, vertical, horizontal,clockwise, and counterclockwise are only used for identificationpurposes to aid the reader's understanding of the present invention, anddo not create limitations, particularly as to the position, orientation,or use of the invention. Connection references, e.g., attached, coupled,connected, and joined are to be construed broadly and may includeintermediate members between a collection of elements and relativemovement between elements unless otherwise indicated. As such,connection references do not necessarily imply that two elements aredirectly connected and in fixed relation to each other. Statedpercentages of light transmission, absorption, and reflection shall beinterpreted as illustrative only and shall not be taken to be limiting.It is intended that all matter contained in the above description orshown in the accompanying drawings shall be interpreted as illustrativeonly and not limiting. Changes in detail or structure may be madewithout departing from the basic elements of the invention as defined inthe following claims.

1. A glass spandrel section for regulating the reflection of radiantenergy comprising a first reflective polarizer; a second polarizer; anda thermotropic depolarizer positioned between the first reflectivepolarizer and the second polarizer that adjusts polarization of incidentlight when below a threshold temperature, wherein above the thresholdtemperature up to 100% of incident light is reflected by the device, andbelow the threshold temperature up to 50% of incident light is reflectedby the device.
 2. A window that regulates the reflection radiant energycomprising one or more panes of glass: a first thermotropic polarizersupported on one of the one or more panes of glass; and a secondthermotropic polarizer supported on one of the one or more panes ofglass, wherein above a threshold temperature down to 0% of incidentradiant energy is transmitted by the window, and below the thresholdtemperature up to 100% of the incident radiant energy is transmitted bythe window.